The present invention relates to a novel coring and tooling material for polymer composites. Particularly, the present invention relates to a low-density, water-soluble composite blend used to form a core material for the fabrication of composite parts. In addition, the present invention relates to a low density, water-soluble composite blend used to form a tooling material, where the blend can be tailored to provide a desired coefficient of thermal expansion and thermal conductivity, thus providing a tooling material that is compatible with the composite material used to fabricate the structure.
Composite components are increasingly being utilized in a variety of applications due to their high strength-to-weight and high stiffness-to-weight ratios. One industry in which composite components are used is the aerospace industry. Initially, composite components were limited to secondary structures such as floorboards and engine cowlings due to limited experience with designing composite structures. However, as the mechanics of composite materials became better understood and higher quality materials were developed, their use increased as primary aircraft components such as flaps, wing sections, and even as the entire fuselage.
Currently, there exist commercial aircraft that have a completely composite fuselage and wings made entirely from composite materials. Commercial airline manufacturers have increased their dependence upon composite materials to meet their ever-increasing demands for improved efficiency and lower costs. Composite materials also are used in military and defense applications, where the performance requirements may be even more demanding. A significant drawback to the use of composite structures in aerospace applications, whether commercial or military, is the complicated and expensive tooling that is required for their fabrication. Many different processes exist for the fabrication of composite structures, and many different demands are placed upon tooling designs and materials. Typically, a composite structure is fabricated using either a closed or an open mold system. In a closed mold system, dimensional accuracy is required for both sides of the composite component. A composite structure of this type would be, for example, an aileron or flap, of sufficient thickness to allow the desired aerodynamic shape to be formed on both sides. Alternatively, an open mold process can be utilized to fabricate parts such as engine cowlings because only one surface, the outer surface (thus, the mold surface), is of importance. With either mold system, the tool gives the composite structure its final shape.
Tools for composite structures can be fabricated from a variety of materials. However, several factors must be considered in the tool design. For instance, the coefficient of thermal expansion of the mold material is of fundamental importance. As the tool is heated, it may change shape at a different rate than the composite materials if the coefficients of the tool and composite material are not similar enough. At elevated temperatures the composite material becomes rigid, whereas, when it is cooled, it will contract. The difference in the coefficient of thermal expansion of the composite and of the tool can create geometrical inaccuracies as well as residual stresses.
Another important factor to consider is the thermal conductivity of the tool material. If the tool material has a low thermal conductivity, significant time can be spent simply getting sufficient heat to the composite part. Thus, curing irregularities can develop between areas of thick and thin tooling. These irregularities also translate into geometric inaccuracies and residual stresses.
Given these restrictions, tools for composite structures are most often comprised of steel, invar, aluminum, and carbon/BMI. With the exception of invar and carbon/BMI materials, the tooling materials generally have a much higher coefficient of thermal expansion than the composite material being fabricated, and this expansion must be accounted for in the mold design. Also, metal mold materials generally require complex and time-consuming machining operations in order to create the tool surface, which further contributes to design complexities. For larger components, the time required to generate the surface of the tool can become unacceptable. Additionally, it can be very difficult to make any modifications to metal tooling once made, if changes to a part are subsequently identified. Thus, if part changes are required, it is often easier to make new metal tooling rather than attempt to re-work the original tooling.
Although composite-tooling materials may seem ideal due to the matched coefficient of thermal expansion, such tooling requires another complex composite component fabrication cycle for the tool itself. Furthermore, a higher processing temperature for the composite structure requires higher cure temperatures for the tool material. Generally, this results in the use of thermoplastic tooling systems that are difficult and expensive to work with.
Use of mandrels made of polymeric binder compositions to form rocket motors, housings and other uniquely shaped items is known. For example, U.S. Pat. No. 6,325,958, which is incorporated by reference herein, discloses methods of manufacture of a mandrel from a mixture that includes water-soluble organic binders. More specifically, the preferred binder comprises, poly (2-ethyl-2-oxazoline), derivatives of poly (2-ethyl-2-oxazoline) and mixtures thereof, along with polyvinylpyrrolidone, derivatives and copolymers of polyvinylpyrrolidone and mixtures thereof. Poly (2-ethyl-2-oxazoline), also referred to as xe2x80x9cPEOxe2x80x9d or xe2x80x9cPEOx,xe2x80x9d tends to be a relatively high cost component. Additionally, the functional properties of PEOx, such as its glass transition temperature, may not be compatible with certain composite formulations for the parts made using the mandrels.
Other conventional materials used for making tooling such as mandrels include eutectic salt, sodium silicate-bonded sand, and poly(vinyl alcohol) bonded ceramic microspheres. These materials pose certain processing problems associated with removal of the materials from the cured parts, as well as with the disposal of the materials. Eutectic salt mandrels are heavy (xcfx81 greater than 2 g/cc) and have high lineal thermal expansion (xcex1 greater than 6xc3x9710xe2x88x925Kxe2x88x921). Furthermore, salt mandrels are brittle and must be cast into the desired shape while molten to avoid machining them with diamond tooling. Despite being soluble in water, eutectic salt mandrels produce corrosive, environmentally unfriendly waste streams when washed from the cured composite part. Sodium silicate-bonded sand mandrels are readily washed from the cured composite and do not produce corrosive waste streams. Unfortunately, silicate-bonded mandrels are heavy and brittle, making them difficult to machine without resorting to diamond tooling. Mandrels made from ceramic microspheres bonded together by poly(vinyl alcohol) have low densities and form relatively easily but have a limited range of temperatures between which they can be used, because poly(vinyl alcohol) polymer binder becomes crosslinked above 200xc2x0 C., making it difficult to wash the mandrel from the cured part.
Thus, there remains a need for compatible, cost-effective, water-soluble compositions for use as coring and tooling materials in the fabrication of composite parts.
The present invention offers alternative coring and tooling system and materials. The present invention offers novel low-cost coring and tooling materials for composite parts. Unlike conventional coring and tooling materials, the materials of the present invention are readily soluble in water and can easily be washed away from the finished part. Furthermore, the coring and tooling materials can be used in the manufacture of a wide range of composite parts that can be cured at higher temperatures than heretofore possible.
Accordingly, an object of the present invention is to provide a composite coring and tooling material that is cost-effective, environmentally benign, and water-soluble.
Another object of the present invention is to provide coring and tooling materials that can be easily shaped and subsequently removed from cured composite parts.
Yet another object of the present invention is to provide composite coring and tooling materials that are strong and lightweight yet capable of withstanding high curing temperatures.
Furthermore, an object of the present invention is to provide tooling materials that can be tailored to provide a specific coefficient of thermal expansion and thermal conductivity, thus providing tooling materials that can be matched to the composite structure being fabricated.